1. Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide semiconductor device including an ohmic electrode.
2. Description of the Related Art
When a vertical power device is formed in a silicon carbide (SiC) substrate and when an electrode for coupling the vertical power device and an electric circuit, in particular, a drain electrode is formed, an ohmic electrode that reduces a contact resistance between the SiC substrate and the drain electrode can be formed.
A method of manufacturing an SiC semiconductor device including an ohmic electrode is disclosed, for example, in Imai et al., “N-type and p-type ohmic contacts for 4H-SiC using Ni salicide process”, 29p-ZM-14, proceedings of the 51st Meeting, the Japan Society of Applied Physics and Related Societies, Mar. 28, 2004. In the manufacturing method, a nickel (Ni) silicide layer is formed on the SiC substrate by a silicide process in order to form an ohmic electrode that maintains a low resistance contact with both of an n type SiC and a p type SiC. The silicide process includes performing vacuum evaporation of Ni on the SiC substrate and then performing a thermal treatment of the SiC substrate.
JP-A-2004-158702 discloses a method including forming an impurity-doped layer on an SiC substrate, forming a metal thin layer on the impurity-doped layer, and irradiating the metal thin layer with a laser light to form an ohmic electrode.
Specifically, after an electrode is formed on a front surface of the SiC substrate, the electrode on the front surface is protected with a resin layer. Then, a thickness of the SiC substrate is reduced from a rear surface, and impurity ions are implanted into the rear surface of the SiC substrate. After activating the impurities by a high-temperature heat treatment, the metal thin layer as an electrode is formed on the rear surface of the SiC substrate. The metal thin layer is irradiated with the laser light, and thereby the ohmic electrode is formed.
JP-A-2006-41248 discloses a method of forming an ohmic electrode after forming a fine asperity on an exposed surface of an SiC substrate by polishing or irradiating with a laser light.
Specifically, after an electrode is formed on a front surface of the SiC substrate, the electrode on the front surface is protected with a resin layer. A thickness of the SiC substrate is reduced from a rear surface, and the rear surface of the SiC substrate is treated with a polishing process or a laser irradiation so that the fine asperity is formed on the rear surface. Then, a metal thin layer as an electrode is formed on the rear surface on which the fine asperity is formed.
In the method disclosed in JP-A-2006-41248, Ni is used as a material of the Ohmic electrode. Thus, a sintering over 800° C. is required for forming Ni silicide that is a compound of Ni and Si in SiC.
In the method disclosed in JP-A-2004-158702, an impurity-doped layer is formed on the rear surface of the SiC substrate before irradiating the rear surface with the laser light. In order to activate the impurities in the impurity-doped layer, a heat treatment of the SiC substrate at a relatively high temperature is required after forming, the impurity-doped layer. In an ion implantation method, a heat treatment of the SiC substrate is performed, for example, at a temperature from 1600° C. to 1700° C.
Thus, in the above-described methods, the electrode on the front surface of the SiC substrate may be damaged during the heat treatment, and various failures may occur in a device.
In a device in which electric current flows in a front-rear direction such as a vertical power device, it is preferred that a thickness of an SiC substrate is reduced for reducing an operation resistance. However, when the thickness of the SiC substrate is too small, it is difficult to perform a high-temperature heat treatment of the SiC substrate and to form an ohmic electrode on a rear surface of the SiC substrate.
As a method of activating an impurity-doped layer without a high-temperature heat treatment, JP-A-2002-289550 discloses a method of irradiating an SiC substrate with a laser light.
Specifically, an electrode is formed on a front surface of SiC substrate in which a vertical element is formed. Next, the front surface of the SiC substrate is protected with a resin film, and a thickness of the SiC substrate is reduced from a rear surface of the SiC substrate. Then, impurity ions are implanted into the rear surface of the SiC substrate, and the rear surface is irradiated with the laser light. After that, a metal thin layer is formed on the rear surface of the SiC substrate.
The method including the ion implanting process has difficulties that ion implanting process is expensive in addition to a fact that an ion implanting apparatus is expensive. Therefore, it is to be desired that an ohmic electrode is formed without an ion implanting process.
As a method not using an ion implanting process, JP-A-2008-135611 discloses a method of forming a metal layer on an SiC substrate and irradiating the SiC substrate with a laser light.
Specifically, a rear surface of the SiC substrate is polished to form an asperity having a surface roughness (Ra) of from 10 nm to 50 nm and the metal layer is formed on the rear surface. Then, the rear surface of the SiC substrate is irradiated with the laser light, and thereby an electrode is formed.
In the above-described method, it is difficult to perform a photo process due to the asperity. Furthermore, when a thickness of the SiC substrate is reduced, a crack may occur due to damage by polishing. Thus, it is to be desired that an ohmic electrode is formed without forming an asperity by polishing.